Selective etching is part of essentially all currently known commercially significant methods for the manufacture of semiconductor devices, e.g., discrete or integrated electronic, opto-electronic, or optical semiconductor-based devices. Typically, selective etching is achieved by means of a suitably patterned processing layer that permits exposure of predetermined regions of a semiconductor member (frequently referred to as a "wafer") to an appropriate etching medium. The processing layer typically is patterned by a process that comprises exposure of predetermined regions of the layer to actinic radiation, whereby the chemical properties of the exposed processing layer material are altered, permitting selective removal of portions of the processing layer. This process generally is referred to as "lithography".
It is well known that individual semiconductor devices are being made progressively smaller, with the number of devices in integrated circuits (ICs) becoming progressively larger. Those skilled in the art generally speak of the size of a device in terms of the "design rule", a typical dimension of the device. Design rules of less than 1 .mu.m are already commercialized, and it is anticipated that design rules of less than 0.5 .mu.m will be used commercially within the next few years. It appears almost inevitable that the trend towards smaller and smaller devices will continue for some time to come, due to the technological and cost advantages that follow from the use of ICs that have larger and larger device counts. One of the technological advantages is higher speed due to shorter transit times of the carriers.
Currently the actinic radiation used in lithography is in the visible or UV part of the electromagnetic spectrum. As is well known, the obtainable feature size depends on the wavelength of the radiation used, with smaller design rules requiring the use of shorter wavelength actinic radiation for lithography. It is anticipated that current optical lithography techniques will not be usable for design rules below about 0.25 .mu.m. Instead, it will be necessary to use actinic radiation of shorter wavelength. This could be particle beams, e.g., electron beams, or short wavelength electromagnetic radiation, namely, X-radiation. This application deals with apparatus for X-ray lithography.
Apparatus for conventional semiconductor lithography typically comprises a source of actinic radiation, refractive or catadioptric (i.e. containing both refractive and reflective elements) optics (using glass or quartz refractive elements) for conditioning the beam, and means for positioning a "mask" between the radiation source and the wafer that is to be exposed. Many commercially available optical lithography systems are of the step-and-repeat type. Such systems can be of the reducing or non-reducing type. In systems the former type the image feature size is smaller than the corresponding mask feature size, the size relationship being expressed in terms of the reduction ratio.
In view of the anticipated exceedingly small design rules it would be desirable to have available lithography apparatus of the reducing type, such that the mask features can be larger than the corresponding features on the wafer, making mask production easier. Furthermore, it would be desirable if the projection system of the apparatus had no image field curvature, such that both a flat mask and a flat wafer could be used. Still furthermore, it would be desirable if the projection system could produce a relatively large image field to avoid the need for scanning, and if the system could have resolution better than anticipated design rules over a relatively large two-dimensional image field. "Resolution" is defined herein as the minimum line width of a periodic square wave pattern which can be imaged by the projection system with contrast .gtoreq.0.65. Finally, it would be highly desirable if the projection system were telecentric in the image space. By this is meant that the chief rays are parallel to each other in the image space. This generally greatly reduces the criticality of wafer positioning. This application discloses a novel X-ray lithography apparatus that can possess these and other desirable features.
The art knows several optical systems (generally four-mirror systems that were typically designed for use at optical or IR wavelengths) which have some of the attributes which are essential for an X-ray exposure system. However, no prior art system successfully meets all of the most important requirements. In particular, no prior art system overcomes the problems presented by the need for a wide (e.g., circular or square) image field that is telecentric in the image space. Among the chief shortcomings of the relevant prior art systems, in addition to the fact that they are designed for use at much longer wavelengths than those that are to be used in X-ray lithography, are the, typically, high aspect-ratio of their image field and/or their curvature of field. Because of these and other shortcomings, prior art systems are not appropriate for use in an X-ray lithography system of the type that is relevant to this application.
One example of a state-of-the-art unobscured four-mirror optical system is disclosed in U.S. Pat. No. 4,226,501, and D. R. Shafer, Applied Optics, Vol. 17(7), pp. 1072-1074. The prior art system combines a backwards Schwarzschild system with a known ring field system, resulting in a four mirror system with five reflections which is corrected for spherical aberration, coma, and astigmatism. However, in the prior art system the field curvature is not corrected. Furthermore, the prior art system is apparently designed for operation in the visible and/or IR part of the spectrum, has a rather small usable two-dimensional field, but can be used to provide a wide angle ring field or a moderately wide angle strip field.
Another example of a relevant four-mirror system is disclosed in abstract TuB3 of the 1983 Annual Meeting of the Optical Society of America. The author of the abstract is D. R. Shafer. The system described in the abstract is said to have a flat image field, in addition to being corrected for spherical aberration, coma and astigmatism. It is said to be well corrected for ring or strip image field shapes. The system is also a telescope system, and apparently is designed for use in the visible and/or IR part of the spectrum.
As is well known, a reflective optical system that is designed to meet certain performance criteria (e.g., a predetermined resolution over some given image field size) at a given wavelength can in general not be adapted for operation at a significantly shorter wavelength by mere dimensional scaling of the original design, since the usable field size will be scaled by the same factor as the wavelength. Thus, a design that meets certain resolution and aberration criteria (expressed as a fraction of the operating wavelength) at wavelength .lambda. over a certain size image field will in general not meet the same criteria (expressed as the same fraction of a new operating wavelength .lambda.') over the same size of the image field if .lambda.' is much shorter than .lambda. (e.g., .lambda.'/.lambda..ltorsim.0.2). For example, if an all reflective system has peak-to-valley wavefront aberration .ltoreq.0.25.lambda. over a certain field size (such a system is well corrected and said to be diffraction-limited over the field size), the same system would have a wavefront aberration of .ltoreq.2.5.lambda.', when operated at a wavelength .lambda.' =.lambda./10. The system thus would not be well corrected for the shorter wavelength, and would not be diffraction limited.